Method of corrosion protection at pipe junctions

ABSTRACT

A method for providing protection against corrosion of a pipe joint (1) formed by the mating of complementary ends (4, 5) of two pipes comprising the steps of: 
     (a) applying controlled heating to a pipe body which is to be joined to an adjacent pipe, 
     (b) applying to the ends (4, 5) of each pipe a protective layer or coating of material 2 to the pipe body by a time/temperature function process, 
     (c) allowing the pipe to cool, 
     (d) bevelling the terminations of the protective layer or coating (2), 
     (e) mating complimentary ends (4, 5) of the pipes together to form a sealed and corrosion resistant joint (1).

The present invention relates to a method of producing rubber-ring steeljointed pipe junctions having high internal and external long termcorrosion protection, tight pipe end tolerances and being capable ofwithstanding high operating pressures. Whilst the invention is accordingto one embodiment directed towards and primarily described for producingsteel pipe to be used in underground pipelines, it is anticipated thatthe invention is suitable for other forms of pipelines such asoverground installations.

Earlier corrosion protection methods for the outer surface ofunderground pipelines have utilized bituminous enamels, whilst morerecently the application of high quality plastic coatings such aspolyethylene (PE) have been used. Both these methods of corrosionprotection have been used in conjunction with cathodic protectionsystems. Typical prior art corrosion protection methods also utilizeepoxy or cement linings for the internal surface of the steel pipe.

Corrosion protection utilizing coatings such as bituminous coatings havethe major disadvantage that they create an effect known as the areaeffect, resulting in accelerated local corrosion if the coating isdamaged. This often occurs as a result of abrasion or impact duringlaying of pipes.

Generally, a weak point in a pipeline occurs at the junction of pipesdue to the fact that it is a site where the necessary elements to createelectrolytic corrosion are present. It is critical that pipe joints beprovided with effective resistance against corrosion however, in thepast this has proved to be difficult.

At pipe joints it is necessary to provide &means for the purpose ofpreventing ingress of moisture which may precipitate corrosiveprocesses. Furthermore, it is necessary to provide means to prevent thesetting up of any electrolytic corrosion process. Pipe joints typicallycomprise the mating of a socket end (female part) and a spigot end (malepart).

The socket end preferably has an internal groove into which a rubberring fits to effect sealing of the joint.

Typically, the rubber-ring socket end of the pipe has previously beenuncoated causing further major problems associated with use of theabove-mentioned corrosion protection methods. The failure of such pipesresulting from corrosion of the pipe ends arises from the ingress ofmoisture and corrosive agents at the pipe joint.

In order to overcome the problems associated with the corrosion of pipeends, the use of plastic coatings on the outer surface of the pipe hasbeen extended to cover the internal surface of the rubber ring socketend of the pipe. Even though such practice has marginally improved thecorrosion protection for such pipes the known methods of producingrubber ring socket ended steel pipe have not been adequate for producingeconomical and easy to assemble pipe having the outer plastic coatingextending to the internal surface of the pipe joint. The major drawbacksstem from the critical tolerances required for the efficient mating ofthe pipe joint such that high operating pressures may be withstood andthe increased difficulty associated with laying pipelines having theplastic coating extending to the internal surface of the pipe joint.

The present invention seeks to ameliorate the problems associated withrubber ring socket end joint pipes which have previously utilized anouter plastic coating which extends from the outer surface around thesocket end to the inner surface of the socket end. The present inventiontherefore seeks to provide a method for forming a coated pipe junctionof two pipes having a spigot end married to a socket end, each of saidpipe ends having its outer surface covered with a protective layer,which extends around the socket end providing protection to the innersurface of the socket end. In one broad form according to the methodaspect, the invention comprises the steps of:

(a) hot rolling a pipe socket end at controlled pipe temperature;

(b) quenching the pipe socket end;

(c) partially re-rolling the pipe socket end if it is undersized orrequenching socket end if it is oversized;

(d) grit blasting of the pipe outer surface and the inner surface nearpipe ends;

(e) controlled heating of the pipe;

(f) applying protective layer material to the pipe body by atime/temperature function process;

(g) elimination of any protective layer porosity by post heating;

(h) allowing the pipe to cool,

(i) bevelling the terminations of the protective coating layer.

In another form the present invention provides a method for forming acoated pipe end, said pipe having a spigot end and a socket end, withits outer surface covered with a protective layer, extending from anouter surface around the socket end providing protection to the innersurface of the socket end. The method generally comprises the steps of:

(a) producing a pipe with the spigot end shell having a diameter of apredetermined tolerance;

(b) hot rolling the pipe socket end at first preferred temperaturewhilst pipe is held by at least a first clamp at a first preferreddistance from said socket with the area of the pipe at or near saidclamp being held at a second preferred temperature;

(c) quenching the socket end when said socket end has cooled to a thirdpreferred temperature;

(d)

(i) partially reheating the pipe socket end, if end undersized, to afourth preferred temperature and re rolling,

(ii) partially reheating the socket end if end oversized thence clampingand heating to a fifth preferred temperature level followed byquenching;

(e) grit blasting simultaneously the pipe outer surface and innersurface near the pipe ends in a blast machine;

(f) heating the pipe body to a sixth preferred temperature level withthe pipe body at or near said pipe ends held at a seventh preferredtemperature level;

(g) immersing the pipe into the protective layer material and rotating,with the immersion process being controlled by a time/temperaturefunction;

(h) elimination of the protective layer porosity by infrared radiationpost heating, with the socket inner surface protective layeralternatively heated by induction for pipes having small thicknesses;

(i) cooling the pipe by natural or fan assisted drafting;

(i) bevelling the coating terminations of the protective layer.

In its broadest form the present invention comprises a method for theproduction of a corrosion resistant sealed pipe joint comprisingessentially the steps of:

(a) hot rolling a pipe socket end at a controlled temperature followedby quenching;

(b) grit blasting the inner and outer surface of the pipe particularlyat the end of the pipe;

(c) heating the pipe;

(d) applying a protective layer to the inner and outer surface of thepipe ends;

(e) allowing the pipe to cool;

In the preferred embodiment the pipe coating covers the outer surfaceand inner surface at the pipe ends so that the coating on the innersurface is at least partially sandwiched between the internal pipesurface and a concrete lining. A rubber ring is then inserted in agroove after application of a lubricant in the pipe end to provide aseal between it and a mating pipe.

The process is intended to be completed in the factory so no furtherfield attention to the joint is required.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 illustrates one embodiment of the cross section of the socketended pipe joint prior to assembly.

FIG. 2 illustrates the same embodiment of FIG. 1 fully assembled.

The present invention relates to an improved method for producingrubber-ring steel jointed pipe as shown according to one embodiment inFIGS. 1 and 2.

Referring to FIGS. 1 and 2, two views of a cross section of the pipejoint are shown, the first view showing the joint prior to assembly andthe Becond following assembly. The pipes ends have the coating disposedon their external and internal surfaces as shown.

The pipes are coated externally by a polyethylene coating 2 andinternally lined by a cement mortar lining 3, with the polyethylenecoating 2 and cement mortar lining 3 being provided as corrosionresistant barriers for the steel pipes 1. The spigot end 4 of one pipemay be joined to the socket end 5 of another pipe 1. The socket end 5 isprovided with an internal recess located at position 6 to accommodate arubber ring 8. The rubber-ring 8 is lubricated prior to assembly toassist in assembly of the joint and provides a compressive water-tightseal.

The polyethylene coating 2 extends around the socket 5 end and continuesinternally. The socket 5 end has its inner surface coated with thepolyethylene coating 2 along its length and meets the cement mortarlining 3 which protects the remaining internal length of the pipe 1.

The methodology of the present invention seeks to improve the productionof rubber-ring steel jointed pipes as mentioned above. Joint formationis effected in the following manner.

The pipe is produced initially with the spigot end of the shell tonominal diameters and mechanically sized to mate the socket within ±0.5mm and limiting the growth at a point, 120 mm from the end of the shell.A collapse of 1 plate thickness is introduced at the very end of thespigot to permit ease of entry into the socket during assembly.

Larger growth values will cause assembly difficulties in the field thatcould render the joint impossible to assemble. Diameters below nominalwill reduce rubber pre-compressing which provide the initial seal,controls organic root penetration through the joint, provides roundingforces on the socket to reduce localized lip gaps to below the criticallevel of 2 mm, and therefore eliminates the chance of rubber ringblow-out.

The diametric dimensions of the socket for each specific pipe size aredesigned taking into account the final nominal outside diameter of thespigot and the thickness and tolerance of the corrosion protectioncoating. The socket is hot rolled and this demands precise rolling diedimensions and settings plus an accurately controlled pipe temperatureof 780° C.±20° C. for reproducibility of socket formation.

Since the pipe is held in clamps approx. 250 mm from the end that willbe rolled into a socket, it is imperative that the 780° C.±20° C.temperature be uniform and limited in length to 200 mm with a suddentemperature gradient over the next 50 mm so that the pipe temperature inthe clamp area is held at 400° C. max. This will avoid unacceptableshell deformation during the rolling operation.

The rate of socket material upset must be controlled to 6 mm per rollinghead revolution to ensure concentricity of rolled socket with the pipebody. The socket formation must be completed in 4±1 revolutions of therotary head followed by a controlled unloading speed of the inner die toensure socket roundness and dimensional accuracy.

After the completion of rolling the next step is to quench the pipe.While the pipe is still held concentric and round to 0.5 mm by twoclamps (relieved by 1.25% (outer) and 1.00% (inner) to match thetemperature gradient of the shell), the newly rolled socket is waterquenched and shrunk to size. Quenching begins when the pipe temperatureis approx. 450° C.±25° C.

Where sockets are rolled outside specification, they can be corrected asfollows:

When undersized, the correction is carried out by partial re-rollingafter reheating the socket to 75% of the standard rolling temperature.Great care is required when repositioning the already rolled socket intothe die system. The hump must be centered evenly between the outer diesbefore commencing to re-roll.

Oversized product is corrected by reheating the socket to 500° C.±25°C., reclamping and shrinking to size by water quenching.

SURFACE PREPARATION

The next step in the process is to prepare the pipe surface in order toenable application of the corrosion protection layer. Sound adhesionbetween the corrosion protection layer and the substrate is obtainedprincipally due to "Anchor pattern" effects. Optimum pattern conditionsare achieved by the use of steel grit abrasive conforming to "runningmixes" of the following graduation:

    ______________________________________                                               mM Passing                                                                             %                                                             ______________________________________                                               840      12.8%                                                                710      27.0%                                                                600      28.5%                                                                500      14.5%                                                                425       9.2%                                                                355       4.7%                                                                300       3.3%                                                         ______________________________________                                    

TAKEOUT SIZE 177

As 1627 Part 4 Class 2.5-3 with profile height of 50-75 um Rtm and 85-95Rt.

The internal surfaces of the joint ends are prepared simultaneously withthe external process by selective rotational/travel delays when thecritical joint areas are located in the "Hot Spot" region of the blastmachine. Both direct and reflective particle impingement maintainsprofile character in the socket region including the shadow faces.

HEATING

The next step is to heat the pipe to the correct temperature gradientprior to coating the pipe with the protective layer.

Direct propane flame impingement heating with additional and independentopen flame end heaters are located at 6 o'clock providing energy at150,000 KJ/m. Pipe rotation of 7-12 re/min is used during this heatcycle which varies from 4-15 min and is dependent on the pipe mass.Temperature gradients are controlled such that 1-2 m of the pipe endsare held at 40°±5° C. above the pipe body temperature but neverexceeding 400° C., at the time of discharge from the oven.

To balance the pipe end cooling effects, the pipe socket end temperaturemust be held to 30°±5° C. above the pipe body temperature with a maximumof 345° C., when the pipe dipping operation commences.

The spigot end can be held at a temperature closer to the pipe bodytemperature as there is less internal coating applied at that end.

For sound adhesion to be obtained, it is critical that the abovetemperature controls and the following lower limits on dip temperatureare strictly observed:

    ______________________________________                                        280° C. and                                                                          10 mm w.t.                                                      300° C.                                                                              6 mm w.t.                                                       320° C.                                                                              5 mm w.t.                                                       340° C.                                                                              4.5 mm w.t.                                                     ______________________________________                                    

COATING

The coating process step which is a time/temperature function follows.

The pipe is rotated at 5-10 re/min., while immersed to 30% of itsdiameter in a fluidized polyethylene bath, held at 25°-60° C. Highertemperatures increase the rate of fusion onto the pipe. Immersion timesvary between 2.5-5 min. which provides the necessary time to depositcoating thicknesses.

POST HEATING

The porosity in the coating is eliminated in a further step by postheating using infrared radiation techniques varying in time from 5-20min. with the pipe shell temperature being held between 180°-220° C.Radiation levels are held at a point where shell temperature will notdecay more than 1° C./min. and may even rise at a nominal rate of 0.3°C./min.

The socket lining is treated in a similar manner internally except whenshell thicknesses are 5 mm. In this case induction heating is resortedto, using the following conditions:

The induction heating unit operates at 25 kw with 10 kHZ frequency andis ON for a number of seconds for five pipe revolution and OFF fornumber of seconds for one pipe revolutions to hold shell temperatures at230° C.±20° C. Parameters are:

10 mm W.T. Post heat not required.

10 mm W.T. Marginal post heat required.

10 mm W.T. Post heat at 180°-210° C.

Heating limit is below the onset of surface oxidation and crazing.Temperature measurements of coating surface at e=0.96.

COOLING

Cooling of the pipe is the next step and is carried out by natural orfan assisted drafting which lowers the temperature from 200° C.±20° C.to 60° C. within a time limit of not less than 60 minutes.

BUFFING

Buff bevelling of the coating terminations at 1:5 tapers follows thecooling step.

RUBBER RINGS

Rubber rings of a suitable type are utilized for the sealing means ofthe pipe joint.

The present invention therefore provides a method for producingcorrosion barrier coated pipes.

It should be obvious to persons skilled in the art that numerousvariations and modifications could be made to the method and apparatusof the present inventions as described and with reference to thedrawings without departing from the overall scope or spirit of theinvention.

We claim:
 1. A method for providing an extended length of corrosionresistant steel pipe, said length of pipe being formed by the mating ofa socket end first pipe, with a complementary spigot end of a secondpipe and first pipe and said second pipe each having an inner and anouter surface, comprising the steps of:(a) applying controlled heatingto said first pipe which is to be joined to said second pipe, (b)applying to the spigot end and socket end of each pipe and outerprotective layer of a plastic, said protective layer having one of itsends terminating on the outer surface of the pipe and the other endterminating on the inner surface of the pipe such that the protectivelayer covers a portion of the outer surface of each pipe and passescontinuously around to the internal surface of each pipe, (c) heatingthe plastic coated pipe to eliminate protective layer porosity, (d)allowing each pipe to cool, (e) bevelling the plastic protective layerof each pipe, and (f) inserting the spigot end of said first pipe intothe socket end of said second pipe to form a sealed corrosion-resistantjoint.
 2. The method according to claim 1 comprising the additionalpreliminary steps before controlled heating of the first pipe of:(a) hotrolling the first pipe at a first temperature while the first pipe isheld by at least one clamp, (b) quenching the socket end of he firstpipe when the socket end has cooled to a second temperature.
 3. Themethod according to claim 2 further comprising grit blasting the outersurface and the inner surface of each pipe near the pipe ends prior tocontrolled heating of the first and second pipes.
 4. A method accordingto claim 3 further comprising reheating the pipe socket end after saidquenching then rerolling the pipe to increase the size of the pipesocket.
 5. The method according to claim 3 further comprisingreheatingthe socket end after said quenching, then clamping and heating followedby quenching the pipe socket end to reduce its size.
 6. The methodaccording to claim 5 wherein said reheating of the socket end is to atemperature within the range of 475° C. to 525° C.
 7. The methodaccording to claim 2 wherein the hot rolling temperature is controlledwithin the range of 760° C. to 800° C. and at a distance not greaterthan 250 mm from the heated pipe end.
 8. The method according to claim 7wherein the pipe temperature in the clamp area is not greater than 400°C.
 9. The method according to claim 8 wherein the quenching begins whensaid second temperature is within the range of 425° C. to 475° C. 10.The method according to claim 1 wherein said plastic protective layer isapplied by immersing the pipe end in the protective layer material androtating the pipe.
 11. The method according to claim 10 wherein saideliminating of protective layer porosity is by infrared radiation or byinduction heating.
 12. The method according to claim 10 wherein saidcontrolled heating of the first pipe comprises rotating the pipe in anopen flame.
 13. The method according to claim 1 wherein said heating toeliminate protective layer porosity comprises infrared radiation for aperiod of 5 to 20 minutes and at a temperature falling within the rangeof 180° C. to 220° C.
 14. A method for producing protective coating forsteel pipes applied at a spigot end and socket end of each pipe so thatthe coating covers a portion of the outer surface of each pipe andpasses continuously around the pipe extremities and along the internalsurface of each pipe for a predetermined distance thereby providing acorrosion resistant joint formed by mating of the spigot end of one pipewith the socket end of another pipe, the method comprising the stepsof:(a) hot rolling a pipe at a first temperature while the pipe is heldby at least a first clamp at a distance from said socket end with thearea of the pipe at or near said first clamp being held at a secondtemperature, (b) quenching the socket end when said socket end hascooled to a third temperature, (c) grit blasting simultaneously the pipeouter surface and inner surface near the pipe ends in a blast machine,(d) heating the pipe followed by quenching, (e) immersing the pipe intoa protective coating material and rotating the pipe to coat it with alayer of protective coating material, (f) eliminating the protectivelayer porosity by infrared radiation heating or by induction heating,(g) cooling the pipe by natural or fan assisted drafting, and (g)bevelling the coating terminations of the protective layer.
 15. Themethod according to claim 14 wherein the hot rolling temperature iscontrolled within the range of 760° C. to 800° C. at a distance notgreater than 250 mm from the heated pipe end.
 16. The method accordingto claim 15 wherein said second temperature is not greater than 400° C.17. The method according to claim 16 wherein the quenching begins whensaid third temperature is within the range of 425° C. to 475° C.
 18. Themethod according to claim 17 wherein after said quenching of the socketend it is reheated to a temperature within the range of 475° C. to 525°C.
 19. The method according to claim 18 wherein the heating of the pipebefore said immersion in said protective coating material comprisesrotating the pipe in an open flame.
 20. The method according to claim 19wherein said infrared radiation heating takes place for a period of 5 to20 minutes and at a temperature falling within the range of 180° C. to220° C.